Formation of functional neural circuits is critical for proper functioning of the brain. It has been proposed that synaptic connections are refined by neural activity during certain periods of development to establish appropriate neural circuits. Hippocampal circuits consist of highly organized unidirectional synaptic connections: from the entorhinal cortex (EC) to the dentate gyrus (DG) to CA3 to CA1 to the EC. However, despite the well known importance of the hippocampus to memory formation, emotional processing, and social behavior, we know very little about whether and how hippocampal circuits are refined by activity and the developmental periods over which such remodeling occurs. Many forms of mental illness such as autism and schizophrenia are associated with abnormal alterations in the hippocampal circuitry. Thus, the understanding of the manner by which hippocampal circuits are refined should yield novel insights into both the etiology and treatment of these devastating disorders. Here, we have established a genetic system to study the role of activity in the establishment and modification of neural circuits in the mouse hippocampus in vivo. In this system, we conditionally inactivate specific neuronal populations in the memory circuit for restricted periods of time in vivo and examine the effect of inactivation on neural connections. We first examined the refinement of EC-to-DG and DG-to-CA3 connections. Our preliminary results show that when inactivated from embryonic stages, inactive EC and DG axons still reach their appropriate target, but are eliminated during a short period of development by activity-dependent competition with active axons. Our data also suggest that EC and DG axons are refined at different developmental stages. Using this system, we will determine (1) when activity plays important roles for synapse refinement, (2) whether activity suppression during sensitive periods leads to permanent synaptic changes, (3) whether neurogenesis plays a role for synapse refinement in the DG, (4) what specific synaptic changes are involved in synapse refinement, and (5) whether miniature and action potential-triggered neurotransmission play unique roles in synapse refinement. For this, we propose to: Aim 1. Identify the sensitive and critical periods for activity-dependent synapse refinement. Aim 2. Determine the functional consequences of activity suppression during the sensitive periods. Aim 3. Examine the role of neurogenesis in the refinement of DG axons. Aim 4. Investigate what synaptic events take place during synapse refinement. Aim 5. Examine the differential roles for miniature and action potential-triggered neurotransmission in activity- dependent synapse refinement. Through these studies we should understand how functional memory circuits are created in the hippocampus by neural activity. It is anticipated that our research will help us design strategies to prevent and treat mental illness associated with abnormal circuit formation in the hippocampus, such as autism and schizophrenia.